The present invention has been developed in connection with a plant for extracting bitumen from the Athabasca oil sand deposit. At this operation, the oil sands are surface-mined and the contained bitumen is separated from the sand and recovered using what is known as the Clark hot water extraction process ("CHWE"). (The terms "oil" or "bitumen" are used interchangeably herein to identify the hydrocarbon content of oil sand.)
The CHWE process is well known to those in the industry and is described in the patent literature. The "front end" of the process, leading up to the production of cleaned, solvent-diluted bitumen froth, will now be generally described.
The as-mined oil sand is firstly mixed with hot water and caustic in a rotating tumbler to produce a slurry. The slurry is screened, to remove oversize rocks and the like. The screened slurry is diluted with additional hot water and the product is then temporarily retained in a thickener-like vessel, referred to as a primary separation vessel ("PSV"). In the PSV, bitumen globules contact and coat air bubbles which have been entrained in the slurry in the tumbler. The buoyant bitumen-coated bubbles rise through the slurry and form a bitumen froth. The sand in the slurry settles and is discharged from the base of the PSV, together with some water and a small amount of bitumen. This stream is referred to as "PSV underflow". "Middlings", comprising water containing non-buoyant bitumen and fines, collect in the mid-section of the PSV.
The froth overflows the lip of the vessel and is recovered in a launder. This froth stream is referred to as "primary" froth. It typically comprises 65 wt. % bitumen, 28 wt. % water and 7 wt. % particulate solids.
The PSV underflow is introduced into a deep cone vessel, referred to as the tailings oil recovery vessel ("TORV"). Here the PSV underflow is contacted and mixed with a stream of aerated middlings from the PSV. Again, bitumen and air bubbles contact and unite to form buoyant globules that rise and form a froth. This "secondary" froth overflows the lip of the TORV and is recovered. The secondary froth typically comprises 45 wt. % bitumen, 45 wt. % water and 10 wt. % solids.
The middlings from the TORV are withdrawn and processed in a series of sub-aerated, impeller-agitated flotation cells. Secondary froth, typically comprising 40 wt. % bitumen, 50 wt. % water and 10 wt. % solids, is produced from these cells.
The primary and secondary froth streams are combined to yield a product froth stream, typically comprising 60 wt. % bitumen, 32 wt. % water and 8 wt. % solids. This stream will typically have a temperature of 80.degree. C.
The water and solids in the froth are contaminants which need to be reduced in concentration before the froth can be treated in a downstream refinery-type upgrading facility. This cleaning operation is carried out using what is referred to as a "dilution centrifuging circuit".
More particularly, the combined froth product is first deaerated and then diluted with sufficient solvent, specifically naphtha, to provide a solvent to froth ("S/F") ratio of about 0.45 (w/w). This is done to increase the density differential between the bitumen on the one hand and the water and solids on the other. The diluted froth is then processed in a scroll-type centrifuge, to remove coarse solids. The bitumen product from the scroll machine is subsequently processed in a disc-type centrifuge, to remove water and fine clay solids.
The "cleaned" bitumen product from the dilution centrifuging circuit typically contains 3 to 5 wt. % water and about 0.6 wt. % solids.
The underflows from the TORV, the flotation cells and the dilution centrifuging circuit are discharged as tailings into a pond system. Water is recycled from this pond for use as plant process water.
There are two significant problems associated with producing a cleaned diluted froth still containing such quantities of water and solids. Firstly, one is precluded from shipping the product through a commercial pipeline that is conveying discrete shipments of a variety of hydrocarbon products. Such pipelines require that any product shipped must contain less than 0.5 wt. % B S and W (bottom settlings and water). Because of this requirement, one must upgrade the cleaned diluted froth produced by the dilution centrifuging circuit in a refinery-type upgrading circuit located close to the mining site, before shipping it. Providing and operating an upgrading circuit at the mine site is very expensive. Secondly, there is a build-up in the concentration of chlorides in plant process water that occurs over time. This build-up arises from recycling water from the tailings pond to the tumbler and re-using the tailings water as part of the water used as process water. In addition, the incoming oil sands contain salt which adds to the chloride content in the process water. Keeping in mind that the cleaned diluted bitumen product from the dilution centrifuging circuit contains a significant fraction of plant water, chlorides are brought by this fraction into the upgrading circuit. These chlorides are harmful in the upgrading circuit, as they cause corrosion and catalyst fouling.
The industry has long understood that it would be very desirable to produce a dry diluted bitumen froth product containing less than about 0.5 wt. % water plus solids. Stated alternatively, it would be desirable to separate substantially all of the water and solids from the froth.
Many potential solutions have been explored. These have included electrostatic desalting, water-washing, chemicals addition, third stage centrifuging and high temperature froth treatment. However, no effective and practical technique has yet emerged which would produce dry bitumen with little accompanying bitumen loss with the water.
There are various reasons why no successful technique has yet been devised for cleaning bitumen froth to reduce the water plus solids content below 0.5 wt. %. The major reason is that the water remaining in naphtha-diluted bitumen froth is finely disseminated in the bitumen as globules having a diameter of the order of 3 microns or less. The mixture is an emulsion that tenaciously resists breakdown.
In this background, only the CHWE process has been mentioned. There are other water extraction processes--such as the known OSLO process, the Bitmin process, and the Kryer process--which also produce bitumen froth which can be cleaned by this invention.
With this background in mind, it is the objective of the present invention to provide a new method for cleaning bitumen froth, produced by a water extraction process, which method is effective to better reduce the water plus solids content, preferably to about 0.5 wt. % or less.